wcs: don't crash indexing an unconfigured category cell (#20975)

combo/apps/wcs/models.py

@@ -395,6 +395,8 @@ class WcsFormsOfCategoryCell(WcsCommonCategoryCell, WcsBlurpMixin):
         return ''
 
     def get_external_links_data(self):
+        if not self.category_reference:
+            return
         formdefs = self.get_data({'synchronous': True})
         for site in formdefs.values():
             for formdef in site.get('data'):